An autopsy case of systemic histiocytosis with excessive deposition of K‐type light chain crystals was reported in a 58 year‐old man who had consistently showed K‐type light chain paraproteinemia, Bence Jones proteinuria and hypogammaglobulinemia for about 10 years until his death. However, no bony destruction was found by repeated X‐ray examinations. At autopsy, extensive hyperplasia of crystal‐storing histiocytes was observed in the bone marrow, spleen, liver, lymph nodes, interstitial tissues of visceral organs and loose connective tissues. In the bone marrow and some other tissues, mild proliferation of plasmocytoid cells containing small crystals were found, Histochemically the crystals positively stained with various methods for amino acids and proteins, especially with Weigerts’method for fibrin Ultra‐structurally intralysosomal crystal deposition was confirmed in the storage histiocytes and derivation of the crystals from Golgi's sacculi in the plasmocytoid cells was suggested. Biochemically the crystals were regarded as mainly consisting of dimers of a variable half of light chain immunoglobulin and immunochemically and immunohistochemically reacted to anti‐ type light chain serum. Such a generalized storage histiocytosis may be secondarily induced by immunoglobulin synthesized in pla